The persistence of plastics in marine ecosystems is attracting an increasing amount of attention, and rightly so! The robust nature of synthetic material poses a considerable threat to marine life, primarily via animal entanglement, ingestion of plastics, and subsequent bioaccumulation. Previous research has shown that half of all sea turtles in the world have already ingested plastic debris (and some research suggests this could be even higher).

Measured values do, however, vary considerably between regions: 90% of juvenile green turtles in the South West Atlantic show evidence of plastic ingestion, while the proportion for juvenile loggerhead turtles in the Western Mediterranean is 80%, and 100% of turtles surveyed in coastal Brazil show evidence of plastic ingestion. Recent analyses suggest that plastics which appear similar to turtles’ natural foods are ingested at higher rates. Additionally, ingestion rates may increase when natural food resources become compromised.

Despite this overwhelming knowledge, no quantitative relationship between the amount of plastic ingested by a turtle and the direct consequences for the individual has been established. This is at least partially due to the difficulty of running controlled trials that are typically used to establish a dose-response relationship between a given contaminant and its consequences.

In this study, a group of Australian scientists decided to start filling in this knowledge gap. They tested the relationship between the amount of plastic pieces turtles have ingested and the likelihood of their deaths. For this they used two datasets; one based on necropsies of 246 sea turtles and a second using 706 records extracted from a national strandings database.

Of the examined turtles, a quarter contained plastic debris in their gastro-intestinal tracts. The count of pieces present ranged from a single piece to 329 pieces per individual. As expected, animals that had ‘plastic ingestion’ listed as the cause of death had higher concentrations of debris than animals that died of known non-plastic ingestion related causes. Using statistical analysis, the team was able to predict the relationship between the load of plastic in the gut and the probability of death due to plastic ingestion. The magic number was 14 items. In other words, 14 pieces of ingested plastic correspond to a 50% probability of mortality.

However, the findings also point out that animals can die even when they have ingested a single item. In two separate cases, single pieces of plastic caused mortal gut perforation and critical gut impaction. All in all, the results suggest there is at least a 22% chance that a turtle will die due to ingesting a single debris item, exposing what a huge risk all marine turtles are exposed to in the anthropocene. This is supported by recent research that has found that all seven species of marine turtles ingest macroplastics (pieces larger than 5 mm).

The most exciting outcome from this study is the ability to predict the mortality probability for any turtle, given his/her size, age, and number of ingested plastic items. This is very important for turtles: due to their biological inability to regurgitate items and the complexity of their gastrointestinal tract geometry, they are particularly prone to accumulation of debris. One feeding experiment showed that rather than passing through the turtles individually, pieces of soft plastic can aggregate and pass as a single compacted item, despite being ingested at different times. The potential to eventually form obstructions is high – it was noted that it can take up to four months for small pieces of soft latex and up to six months for plastic sheets to be eliminated.

Animal advocates will certainly find this effort uplifting – the known impact of plastic pollution will add weight to ongoing advocacy efforts.There is a critical link between the amount of plastic ingested and the effect that this has on populations. Now, it’s up to advocates to use this information as soon as they can, as plastic ingestion by marine animals is an ongoing problem that needs immediate action.

Lukas Jasiunas is an active animal advocate and proponent of science. He is currently a PhD candidate in the field of Chemical Engineering - studying ways to replace petroleum products by making use of widely available residual biomass.